1. Field of the Invention
The present invention relates to an apparatus for forming a transparent conductive film by sputtering and a method therefor, and more particularly to an apparatus and method capable, in forming a transparent conductive film by sputtering, of preventing destruction (shortcircuiting) of the semiconductor junction layer and reducing the damage thereto by plasma.
2. Related Background Art
The configuration of the conventional photovoltaic element consists, for example, of a reflective layer, a semiconductor junction layer and a transparent conductive film formed in succession on a conductive substrate.
The transparent conductive film mentioned above is often formed by evaporation or DC magnetron sputtering. The evaporation method, generally having a low deposition rate for the transparent conductive film, is not suitable for mass production. On the other hand, the DC magnetron sputtering is widely used because a high deposition rate can be easily obtained.
FIG. 6 shows an example of the DC magnetron sputtering apparatus for depositing the transparent conductive film on a continuous conductive substrate. In the apparatus shown in FIG. 6, a conductive substrate 1101, bearing thereon a reflective layer and a semiconductor junction layer in succession, moves through a vacuum chamber 1104 by a roll-to-roll system. In this operation, the conductive substrate 1101 is maintained at the ground potential, same as the potential of the vacuum chamber 1104, and serves also as an anode electrode. The semiconductor junction layer, at the outermost surface of the conductive substrate 1101, is the surface receiving the deposition Lr, of the transparent conductive film and directed to a cathode electrode 1106. In FIG. 6 there are also shown rollers 1102, 1103, 1102a, 1103a, a target 1105, a cathode electrode 1106, a magnet 1107, a DC power source 1108, a voltmeter 1109, a heater 1110, and a sensor 1111. The vacuum chamber 1104 is composed of a substrate feeding chamber 1121, a film forming chamber 1122 and a substrate take-up chamber 1123.
The transparent conductive film is formed on the semiconductor junction layer by applying a predetermined DC voltage between the cathode electrode and the anode electrode, thereby sputtering the target material, placed on the cathode electrode, for forming the transparent conductive film.
However, in case of depositing the transparent conductive film on the conductive substrate by the above-mentioned apparatus for forming the transparent conductive film by sputtering, a DC current according to the output of the DC power source is generated in the conductive substrate serving also as the anode electrode, from the semiconductor junction layer at the outermost surface toward the conductive substrate. The current is concentrated in an electrically weak portion of the semiconductor junction layer, thereby causing destruction (shortcircuiting) in such portion. As a result, the photovoltaic element becomes incapable of performing its function.
Also if the self bias voltage Vself of the continuous conductive substrate becomes excessively negative, the semiconductor junction layer receives increased plasma damage by cations such as Ar.sup.+, whereby the photovoltaic element becomes incapable of performing its function.
The self bias voltage means a potential difference generated between the conductive substrate and the vacuum chamber, and more specifically the potential of the conductive substrate when the potential of the vacuum chamber is taken as 0.